Over the past decade, the population of the United States rose by 27.3 million. The 2010 Census found that 13.9 million of the nearly 40 million foreign-born residents had arrived since 2000. That identifies new immigration – both legal and illegal – as amounting to more than half of the nation’s population increase over the decade. However, the immigrant population overall has increased during the same period by a lower amount – 9.05 million – because many of earlier immigrants have either died or left voluntarily or involuntarily.

In addition to that direct effect of immigration on population increase, immigration also adds to the population through the birth to immigrant mothers. The immigrant mothers have about twice as large a share of births as their share of the overall population, which implies that they accounted for about 9.6 million births over the decade. That indirect effect of immigration when added to the net increase in the immigrant population means that over the past decade immigration accounted for more than two-thirds (68.3%) of the nation’s population increase.